Interpretive Summary: New breeds of cattle called composites are formed from crosses of existing breeds to optimize performance levels and use heterosis. Composites may have either more or less genetic variation that is useful for genetic improvement compared with the purebreds making up the composite. The average size and milk level of a breed or composite also can affect the amount of genetic variation in weight measurements. Genetic variation in birth weight, weaning weight, and postweaning gain was estimated for three composites and nine parental breeds. Mating system (composite vs. purebred), size, and milk were evaluated for their effects on genetic variation. Heavier average weights resulted in more genetic variance for weight measurements. Higher average milk production resulted in greater genetic variation in weaning weight being attributed to the calf and less being attributed to the influence of the cow. Composites and purebreds had dsimilar phenotypic variation and slightly more genetic variation. However the cow had less influence on weaning weight in composites compared to purebreds. Results suggest that within herd selection in composites should be at least as effective as in purebreds.

Technical Abstract:
Genetic and environmental variances and covariances for birth weight, adjusted 200-d weight and postweaning gain were estimated in nine parental and three composite populations of beef cattle. Heritabilities of additive direct genetic effects for birth weight (.50) and postweaning gain (.49) were greater than 200-d weight (.32). Heritabilities of additive maternal effects of .09 for birth weight and .10 for 200-d weight were much smaller than direct effect heritabilities. Heritabilities of composites were larger than the parental breeds for additive direct effects of all three traits but smaller for maternal 200-d weight. Correlations were high and positive for direct genetic effects of the three weight traits and higher in composites than the parental breeds. Correlations between direct and maternal genetic effects for both birth weight and 200-d weight were near zero. Differences in variances between populations were partially correlated with differences in size and milk yield. Populations of larger size had larger variances. Increased average milk yield was correlated with decreased phenotypic variance of 200-d weight. Average milk yield was also implicated in the expression of direct and maternal genetic effects for 200-d weight and their covariance. Comparison of univariate and multivariate estimates of genetic variances suggested that it is important to include birth weight in multivariate analyses of all weight traits to account for increased preweaning mortality of extremely heavy or light birth weights. Results suggest that within herd selection in composites should be at least as effective as in purebreds. Some differences between populations in genetic parameters are indicated, especially maternal 200-d weight and its correlations with other traits.